posted by: dberry
When I was on DQM night shift last weekend, they (the beam guys) performed a beam scan. A beam scan moves the beam slowly in the horizontal and vertical positions until the maximum collision rate is found. This is done in order to maximize the luminosity for each experiment. The x and y position of the beam is controlled with dipole magnets, and by controlling the current in the magnets they can control the position of the beam. The measurement can be seen in the physics trigger rate (image below).
The valleys are when the beams are out of position and the peaks are when the beams are interacting head on. The sharp jump around lumi-section 200 was because that’s when collisions started. The vertex position also changes during the beam scan. Below is an image of the reconstructed vertex position by the pixel tracker for the last vertical beam scan.
What you see is that when the beam is moved in the y direction the vertex also moves in the y direction. And then when the beam scan is finished, the vertex position is constant.
Tags: Particle Physics